Linear-Combination-of-Atomic-Orbitals Band Structure of TlBr

Valence bands and rough estimates for conduction bands of TlBr are calculated using the linear-combination-of-atomic-orbitals approximation. The calculation is based on potentials and wave functions of the isolated ions obtained from solutions to the Dirac equation with a self-consistent Hartree-Slater potential. The crystal potential is constructed out of effective ionic potentials. In order to investigate the effects of correlation, two types of crystal potentials are constructed, one with the Slater exchange and the other with the Slater exchange modified to simulate the effects of correlation. These effects are found to be quantitatively important; however, the shapes and ordering of the bands are not affected. The band gap is found to be direct, at the $X$ point of the simple-cubic Brillouin zone, and the three-lowest-energy optical-absorption peaks can be unambiguously assigned to transitions from the three-highest-energy-valence-band states at $X$ to a common conduction band. Band masses are calculated at $X$. Expanding the lattice is found to cause an increase in the band gap. The results of the band-structure calculation allow a consistent interpretation of the experimental data.